There's no need for 8 different ways to represent that a type is
non-comparable.

While here, move AMEM out of the runtime-known algorithm values since
it's not needed at run-time, and get rid of the unused AUNK constant.

Change-Id: Ie23972b692c6f27fc5f1a908561b3e26ef5a50e9
Reviewed-on: https://go-review.googlesource.com/19779
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>